The invention relates to a draw roller for strip or web-shaped materials, in particular for webs of paper or card-board or foils of plastic or metal, which has a stationary inner part rotatably supporting an outer jacket that is formed over its entire surface with air holes. The interior of the jacket is subdivided into segment-like chambers from which air is pumped in the region engaging the web. The subatmospheric pressure holds the web on the roller so as to increase the holding strength and thus transmit more tension to the web.
Preferably such draw rollers are used in systems for making and/or finishing web materials (webs of paper or card-board, foils of plastic or metal) in places where the workpiece can only be gripped on one side, for example because the other side has a layer of liquid coating material (German 1,474,973).
With high web-travel speeds the layer of air between the web and the roller creates problems as the web enters into contact with the roller. An air cushion is created between the web and the roller which holds up the web and thus reduces the contact area. This reduction of the contact area means that less tension can be transmitted. In extreme cases the roller rotates relative to the web.
It is an object of the invention to improve on a draw roller of the above-described type so that even at high web speeds large tensions can be transmitted.
A second object of the invention is to provide a draw roller whose surface can easily be adjusted for different web materials, speeds, and/or web widths.
This object is achieved in a draw roller for web-shaped materials, in particular for webs of paper or cardboard or foils of plastic or metal, with a stationary inner part rotatably supporting an outer jacket that is formed over its entire surface with air holes, and with a vacuum chamber maintained at subatmospheric pressure in a region of contact with the web wherein inside the jacket relative to the travel direction of the web directly upstream of the vacuum chamber there is a further vacuum chamber which is maintained at a subatmospheric pressure independent of and greater than that of the chamber.
The upstream second;vacuum chamber makes it possible to eliminate the air trapped between the web and the roller so that the contact area remains large in order to transmit the necessary tension.
The second object is achieved in a draw roller for web-shaped materials with a stationary inner part rotatably supporting an outer jacket that is formed over its entire surface with air holes, the inner part having partitions that subdivide the interior of the jacket into chambers of which at least one chamber is connected to a vacuum line, and wherein that the jacket is releasably mounted at both axial ends on annular rotatably supported bearing parts and can be released and drawn axially off the inner part.